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PSYC2030 - Sensation and Perception Chapter 4(con't)

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PSYC 2220
Jennifer Steeves

Ben Kim PSYC2030 – Sensation & Perception Page 1 of 5 Lecture 4 Basic Principles of Colour Perception • Colour is not a physical property but a psychophysical property o Most of the light we see is reflected o Typical light sources: Sun, fire, light bulb o We see only part of electromagnetic spectrum – between 400 and 700nm A single photoreceptor shows different responses of lights of different wavelengths but the same intensity Trichromacy • Rods are sensitive to scotopic light levels o All rods contain the same photopigment molecule: Rhodopsin o All rods have the same sensitivity to various wavelengths of light o Therefore, rods suffer from the problem of univariance and cannot sense differences in colour o Under scotopic conditions, only rods are active, which is why the world seems drained of colour • Cone photoreceptors: Three varieties: o S-cones: cones that are preferentially sensitive to short wavelengths (“blue” cones) best at 420nm o M-cone: Cones that are preferentially sensitive to middle wavelengths (“green” cones) best at 530nm o L-Cones: cones that are preferentially sensitive to long wavelengths (“red” cones) best at 560nm o Colour blindness, it is not usually for the cone waves to have a medium and long wavelength code (abnormal), mainly M and L on the X chromosome (genetics) o Two colour receptors allow you to see lush colour rather than a flushed out world. Three cones is a slightly less advantage. o Rods are active around 495nm • With three cone types, we can tell the difference between lights of different wavelengths o Under photopic conditions, the S-, M-, and L-cones are all active • The Trichomatic (or Young—Helmholtz) Theory o Colour of any light is defined in our visual system by the relationships of three numbers, the outputs of three receptor types now known to be the three cones • We have much less S-cones (~5%) as compared to the others so it was hard to detect them when research was first done on cones • Metamers – different mixtures of wavelengths that look identical. More generally, any part of stimuli that are perceived as identical in spite of physical differences o Colours that are indistinguishable to the normal visual system Ben Kim PSYC2030 – Sensation & Perception Page 2 of 5  E.g. green and red light mixed together outputs a yellow light • Additive colour mixing: A mixture of lights o If light A and B are both reflected from a surface of the eye, in the perception of colour the effects of those two lights add together • Easiest way to understand colour mixing is mixing colour • If you mix red and yellow together to get orange, the orange colour will be less saturated than the original red and yellow • Subtractive colour mixing: A mixture of pigments o If pigment A and B mix, some of the light shining on the surface will be subtracted by A and some by B. Only the remainder contributes to the perception of colour • Filters can absorb colours with whatever filter it is associated with (blue filter can absorb the colour blue with a light is shined on it and reflect whatever colour is left • Pigments are subtractive and light is additive • However if a white light is shined on a coloured piece of paper (e.g. blue) all other colours will be absorbed other than the blue. o What you see depends on what surface colour and what colour of light is reflected off the surface. Colour Space • Colour space: A three-dimensional space that describes all colour. There are several possible colour spaces • RGB colour space: defined by the outputs of long, medium, and short wavelength lights • HSB colour space: defined by hue saturation, and brightness o Hue: the chromatic (colour) aspect of light o Saturation: The chromatic strength of a hue o Brightness: the distance from black in colour space • Representation of all the colours that can be seen (at one brightness level) by the human visual system Cone Opponent Processes • Lateral Geniculate Nucleus (LGN) has cells that are maximally stimulated by spots of light o Visual pathway synapses in LGN on the way from retina to visual cortex o L
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